examples: apply real texture assets to multi-mesh examples

- meshes/cube: procedural checkerboard -> uv_texture.jpg (8x8 UV grid)
- meshes/pbr: floor -> ground.jpeg, bump cube -> cave.jpg + caveNormal.jpg
  (normal map pre-encoded via sRGB OETF to cancel the GPU sRGB decode)
- lights/shadow: ground -> ground.jpeg, cube -> uv_texture.jpg
- effects/demo: ground -> ground.jpeg, cube -> uv_texture.jpg
- effects/fog: ground -> ground.jpeg (tiled 80x80), cubes -> stonewall.jpg
- effects/dof: ground -> ground.jpeg, cubes -> uv_texture.jpg
- cameras/culling: shared cube mesh -> uv_texture.jpg
- add lib/examples/assets/textures/ (19 assets, 6.5 MB)
- document assets + usage in examples READMEs, docs/user/examples.md,
  docs/user/meshes/materials.md (CARGO_MANIFEST_DIR pattern, sRGB caveat)

lib/examples/cameras/culling.rs

@@ -39,9 +39,13 @@ use wsg_lib::app::AppBuilder;
 use wsg_lib::camera::CameraController;
 use wsg_lib::core::Transform;
 use wsg_lib::mesh::{cube, plane};
+use wsg_lib::resources::Texture;
 use wsg_lib::AppHandler;
 use wsg_lib::utils::WsgError;
 
+/// Texture asset directory, resolved against the crate root so the example works from any CWD.
+const TEXTURES: &str = concat!(env!("CARGO_MANIFEST_DIR"), "/examples/assets/textures");
+
 /// Grid dimensions (15×15 = 225 cubes, fits within MAX_ENTITIES=256).
 const GRID: usize = 15;
 /// Spacing between cubes (world units).
@@ -67,9 +71,26 @@ impl AppHandler for CullingDemo {
             .unwrap();
         app.scene.add_entity("ground", "ground_mesh").unwrap();
 
-        // One shared cube mesh (all entities reference the same GPU buffers).
+        // One shared cube mesh (all entities reference the same GPU buffers), textured with
+        // the uv_texture.jpg atlas — the colourful labelled cells make it obvious exactly
+        // which cubes the GPU draws and which it culls.
+        let (device, queue) = {
+            let ctx = app.context();
+            (ctx.device.clone(), ctx.queue.clone())
+        };
+        let uv_tex = Texture::from_file(
+            &device,
+            &queue,
+            "uv_atlas",
+            &format!("{TEXTURES}/uv_texture.jpg"),
+        )
+        .unwrap();
+        app.scene.add_texture("uv_texture", uv_tex).unwrap();
         app.scene
-            .create_mesh("cube_mesh", cube(0.5), None)
+            .add_material_texture("cube_mat", "standard", "uv_texture")
+            .unwrap();
+        app.scene
+            .create_mesh("cube_mesh", cube(0.5), Some("cube_mat"))
             .unwrap();
 
         // Place the grid of cubes.